Liquid crystal display devices are presently widely used in e.g. displays of personal computers or televisions as display devices which can realize thin structure/light weight. In such liquid crystal display devices, liquid crystal alignment films of a polyamic acid or a polyimide are commonly used in many cases.
Such a liquid crystal alignment film is usually formed by applying a liquid crystal aligning agent having a polyamic acid or a polyimide dissolved in an organic solvent, on a substrate by e.g. flexographic printing, followed by preliminary drying and baking. In such a case, it is undesirable that a coating film of the liquid crystal aligning agent has a partial film thickness irregularity, since such an irregularity may present an adverse effect to the display characteristics of a liquid crystal display device.
When a liquid crystal alignment film is to be formed, selection of the solvent to be used for the liquid crystal aligning agent becomes important in order to form the coating film uniformly without irregularity. As such a solvent, it is common to employ a solvent excellent in the solubility of a polyamic acid or a polyimide in combination with a solvent such as butyl cellosolve. Further, it is known that also in a case where dipropylene glycol monomethyl ether is employed instead of butyl cellosolve, it is possible to obtain a smooth coating film free from irregularities (JP-A-7-109438). Likewise, it is known that also in a case where diethylene glycol diethyl ether is mixed, cissing or coating film thickness irregularity can be reduced (JP-A-8-208983).
Such a liquid crystal aligning agent is, after being applied on a substrate, usually subjected to preliminary drying at a temperature of at least 80° C. Along with the trend for a large size and low costs of liquid crystal display devices in recent years, there is a situation where the preliminary drying of the liquid crystal aligning agent is carried out at a temperature lower than before. However, with a conventional liquid crystal aligning agent, a uniform coating film tends to be hardly formed by drying at such a low temperature, and it has been desired to develop a liquid crystal aligning agent whereby a uniform coating film can be obtained regardless of the temperature for preliminary drying.